Monitoring the Dynamic Process of Formation of Plasmonic Molecular Junctions during Single Nanoparticle Collisions

Plasmon-Molecule Interactions in Single-Molecule Junctions

Single-molecule optoelectronics offers opportunities for advancing integrated photonics and electronics, which also serves as a tool to elucidate the underlying mechanism of light-matter interaction. Plasmonics, which plays pivotal role in the interaction of photons and matter, have became an emerging area. A comprehensive understanding of the plasmonic excitation and modulation mechanisms within single-molecule junctions (SMJs) lays the foundation for optoelectronic devices. Consequently, this review primarily concentrates on illuminating the fundamental principles of plasmonics within SMJs, delving into their research methods and modulation factors of plasmon-exciton. Moreover, we underscore the interaction phenomena within SMJs, including the enhancement of molecular fluorescence by plasmonics, Fano resonance and Rabi splitting caused by the interaction of plasmon-exciton. Finally, by emphasizing the potential applications of plasmonics within SMJs, such as their roles in optical tweezers, single-photon sources, super-resolution imaging, and chemical reactions, we elucidate the future prospects and current challenges in this domain.

Emerging sensing platforms based on Cucurbit[n]uril functionalized gold nanoparticles and electrodes

Cucurbit[n]urils (CB[n]s, n = 5-8, 10, and 14), synthetic macrocycles with unique host-guest properties, have triggered increasing research interest in recent years. Gold nanoparticles (Au NPs) and electrodes stand out as exceptional substrates for sensing due to their remarkable physicochemical characteristics. Coupling the CB[n]s with Au NPs and electrodes has enabled the development of emerging sensing platforms for various promising applications. However, monitoring the behavior of analytes at the single-molecule level is currently one of the most challenging topics in the field of CB[n]-based sensing. Constructing supramolecular junctions in a sensing platform provides an ideal structure for single-molecule analysis, which can provide insights for a fundamental understanding of supramolecular interactions and chemical reactions and guide the design of sensing applications. This feature article outlines the progress in the preparation of the CB[n] functionalized Au NPs and Au electrodes, as well as the construction and application of supramolecular junctions in sensing platforms, based on the methods of recognition tunneling (RT), surface-enhanced Raman spectroscopy (SERS), single-molecule force spectroscopy (SMFS), and electrochemical sensing (ECS). A brief perspective on the future development of and challenges in CB[n] mediated sensing platforms is also covered.

Formation of Molecular Junctions by Single-Entity Collision Electrochemistry

Controlling and understanding the chemistry of molecular junctions is one of the major themes in various fields ranging from chemistry and nanotechnology to biotechnology and biology. Stochastic single-entity collision electrochemistry (SECE) provides powerful tools to study a single entity, such as single cells, single particles, and even single molecules, in a nanoconfined space. Molecular junctions formed by SECE collision show various potential applications in monitoring molecular dynamics with high spatial resolution and high temporal resolution and in feasible combination with hybrid techniques. This Perspective highlights the new breakthroughs, seminal studies, and trends in the area that have been most recently reported. In addition, future challenges for the study of molecular junction dynamics with SECE are discussed.

Plasmonic phenomena in molecular junctions: principles and applications

Molecular junctions are building blocks for constructing future nanoelectronic devices that enable the investigation of a broad range of electronic transport properties within nanoscale regions. Crossing both the nanoscopic and mesoscopic length scales, plasmonics lies at the intersection of the macroscopic photonics and nanoelectronics, owing to their capability of confining light to dimensions far below the diffraction limit. Research activities on plasmonic phenomena in molecular electronics started around 2010, and feedback between plasmons and molecular junctions has increased over the past years. These efforts can provide new insights into the near-field interaction and the corresponding tunability in properties, as well as resultant plasmon-based molecular devices. This Review presents the latest advancements of plasmonic resonances in molecular junctions and details the progress in plasmon excitation and plasmon coupling. We also highlight emerging experimental approaches to unravel the mechanisms behind the various types of light-matter interactions at molecular length scales, where quantum effects come into play. Finally, we discuss the potential of these plasmonic-electronic hybrid systems across various future applications, including sensing, photocatalysis, molecular trapping and active control of molecular switches.

Plasmon-driven photocatalytic properties based on the surface of gold nanostar particles

Surface plasmon resonance (SPR) generated in gold nanoparticles can induce the conversion of p-Aminothiophenol (PATP) molecules into p,p'-dimercaptoazobenzene (DMAB) molecules by coupling reaction under the action of excitation light. Molecular detection of samples by surface enhanced Raman spectroscopy (SERS) techniques allows the study of their plasma-driven photocatalytic reaction processes. In this study, we used gold nanostars (GNS) as the substrate to study its catalytic performance and sensitivity. On this basis, catalytic substrates of gold nanospheres (GNPs) were prepared for comparison. The catalytic reactions of PATP molecules on each of the above two substrates were systematically investigated under 633 nm laser irradiation. The reduction process was subsequently observed by introducing NaBH₄ solution. The results show that photocatalytic reactions can be achieved on both substrates under laser excitation at the same wavelength. However, the catalytic and reduction reaction rates on GNSs as a substrate are much faster than those of GNPs. This phenomenon may be due to the abundant nano-branched microstructures on the surface of GNSs, which will generate more and stronger local surface plasma hot spots under the irradiation of excitation light. In order to test the above hypothesis, the surface electromagnetic field distribution of two nanostructures was numerically simulated using the finite-difference time domain (FDTD) method. It is found that the star-like nanostructures not only have the same inter-particle hot spot system as the spherical nanostructures, but also have a large number of high-intensity single-particle hot spot systems arising from the abundance of branched nanostructures on their own surfaces. Compared with the spherical nanostructures, they are characterized by a dual hot spot system, which accelerates the photocatalytic reaction rate. The above experiments are of some reference significance for the in-depth study of multi-branched nanostructures and surface plasma distribution properties and their applications.

Nanocollision mediated electrochemical sensing of host-guest chemistry at a nanoelectrode surface

Electrochemical (EC) measurements of dynamic nanoparticle collisions on a support electrode provide a powerful approach to study the electrical properties of interfacial molecules self-assembled on the electrode surface. By introducing a special cage-shaped macrocyclic molecule, cucurbit[7]uril (CB7), onto a gold nanoelectrode surface, we show that the dynamic interactions between CB7 and the colliding nanoparticles can be real-time monitored via the appearance of distinct EC current switching signals. When a guest molecule is included in the CB7 cavity, the changed host-guest chemistry can be probed via the amplitude change of the EC current signals. In addition, different guest molecules can be recognized by CB7 on the nanoelectrode surface, giving rise to distinguishable current jump signals for different host-guest systems. Remarkably, two well-defined current states are observed in the EC measurements of the CB7-ferrocene complex, indicating two orientation geometries of ferrocene inside the CB7 cavity can be resolved in this EC sensing platform. This work demonstrates an effective approach for studying the dynamics of host-guest chemistry at the liquid-solid interface and sheds light on a convenient EC sensor for the recognition of target molecules with the aid of CB7.

Direct Observation of Amide Bond Formation in a Plasmonic Nanocavity Triggered by Single Nanoparticle Collisions

The real-time observation of chemical bond formation at the single-molecule level is one of the great challenges in the fields of organic and biomolecular chemistry. Valuable information can be gleaned that is not accessible using ensemble-average measurements. Although remarkably sophisticated techniques for monitoring chemical reactions have been developed, the ability to detect the specific formation of a chemical bond in situ at the single-molecule level has remained an elusive goal. Amide bonds are routinely formed from the aminolysis of N-hydroxysuccinimide (NHS) esters by primary amines, and the protocol is widely used for the synthesis, cross-linking, and labeling of peptides and proteins. Herein, a plasmonic nanocavity was applied to study aminolysis reaction for amide bond formation, which was initiated by single nanoparticle collision events between suitably functionalized free-moving gold nanoparticles and a gold nanoelectrode in an aqueous buffer. By means of simultaneous surface enhanced Raman spectroscopy (SERS) and single-entity electrochemistry (EC) measurements, we have probed the dynamic evolution of amide bond formation in the aminolysis reaction with 10 s of millisecond time resolution. Hence, we demonstrate that single-entity EC-SERS is a valuable and sensitive technique by which chemical reactions can be studied at the single-molecule level.

Effective Electrochemical Modulation of SERS Intensity Assisted by Core-Shell Nanoparticles

An effective and reversible tuning of the intensity of surface-enhanced Raman scattering (SERS) of nonelectroactive molecules at nonresonance conditions by electrochemical means has been developed on plasmonic molecular nanojunctions formed between Au@Ag core-shell nanoparticles (NPs) and a gold nanoelectrode (AuNE) modified with a self-assembled monolayer. The Au@Ag nanoparticle on nanoelectrode (NPoNE) structures are formed in situ by the electrochemical deposition of Ag on AuNPs adsorbed on the AuNE and can be monitored by both the electrochemical current and SERS signals. Instead of introducing molecular changes by the applied electrode potential, the highly effective SERS intensity tuning was achieved by the chemical composition transformation of the ultrathin Ag shell from metallic Ag to insulating AgCl. The electrode potential-induced electromagnetic enhancement (EME) tuning in the Au@Ag NPoNE structure has been confirmed by finite-difference time-domain simulations. Moreover, the specific Raman band associated with Ag-molecule interaction can also be tuned by the electrode potential. Therefore, we demonstrated that the electrode potential could effectively and reversibly modulate both EME and chemical enhancement in Au@Ag NPoNE structures.

Observing dynamic molecular changes at single-molecule level in a cucurbituril based plasmonic molecular junction

In recent years, surface enhanced Raman spectroscopy (SERS) has emerged as a prominent tool for probing molecular interaction and reaction with single-molecule sensitivity. Here we use SERS to investigate the dynamic changes of the cucurbit[7]uril (CB[7]) based plasmonic molecular junctions in solution, which are spontaneously formed by the adsorption of gold nanoparticles (GNPs) at the CB[7] modified gold nanoelectrode (GNE) surface. The typical fingerprint Raman peaks of CB[7] are very weak in the SERS spectra. However, chemically enhanced peaks are prominent in the spectra due to the charge transfer across the metal-molecule interface through specific noncovalent interactions between the gold atoms and CB[7] or its guest molecule. We first investigated the selectively enhanced and greatly shifted C[double bond, length as m-dash]O peak of CB[7] in the SERS spectra. Based on the bias-dependent changes of the C[double bond, length as m-dash]O peak, we found the gold-carbonyl interaction was strengthened by the positive bias applied to the GNE, resulting in stable CB[7] junctions. Next, we found the CB[7] junction could also be stabilized by the inclusion of a guest molecule amino-ferrocene, attributed to the interactions between gold adatoms and the cyclopentadienyl ring of the guest molecule. Because this interaction is sensitive to the orientation of the guest molecule in the cavity, we revealed the rotational motion of a guest molecule inside the CB[7] cavity based on the dynamic spectral changes of the cyclopentadienyl ring peak.

Dynamic single-cell intracellular pH sensing using a SERS-active nanopipette

Glass nanopipettes have shown promise for applications in single-cell manipulation, analysis, and imaging. In recent years, plasmonic nanopipettes have been developed to enable surface-enhanced Raman spectroscopy (SERS) measurements for single-cell analysis. In this work, we developed a SERS-active nanopipette that can be used to perform long-term and reliable intracellular analysis of single living cells with minimal damage, which is achieved by optimizing the nanopipette geometry and the surface density of the gold nanoparticle (AuNP) layer at the nanopipette tip. To demonstrate its ability in single-cell analysis, we used the nanopipette for intracellular pH sensing. Intracellular pH (pHi) is vital to cells as it influences cell function and behavior and pathological conditions. The pH sensitivity was realized by simply modifying the AuNP layer with the pH reporter molecule 4-mercaptobenzoic acid. With a response time of less than 5 seconds, the pH sensing range is from 6.0 to 8.0 and the maximum sensitivity is 0.2 pH units. We monitored the pHi change of individual HeLa and fibroblast cells, triggered by the extracellular pH (pHe) change. The HeLa cancer cells can better resist pHe change and adapt to the weak acidic environment. Plasmonic nanopipettes can be further developed to monitor other intracellular biomarkers.

Observing Host-Guest Interactions at Molecular Interfaces by Monitoring the Electrochemical Current

Macrocyclic cucurbit[n]uril (CB[n]) molecules have triggered renewed interest because of their outstanding capabilities as host molecules to selectively interact with a wide range of small guest molecules. Here, CB[7]-based host-guest interactions were investigated for a guest-modified nanoelectrode by monitoring the electrochemical current. A ferrocene (Fc)-terminated molecule immobilized on a gold nanoelectrode (GNE) showed suitable affinity with CB[7] when the effective exposing area of the GNE was between 5.3 and 12 μm² and the bias applied on the GNE was -500 mV. Monitoring the dynamics of nanoparticles (NPs) on a nanoelectrode provides new insights into the host-guest interactions at molecular interfaces.

Plasmonic Measurement of Electron Transfer between a Single Metal Nanoparticle and an Electrode through a Molecular Layer

We study electron transfer associated with electrocatalytic reduction of hydrogen on single platinum nanoparticles separated from an electrode surface with an alkanethiol monolayer using a plasmonic imaging technique. By varying the monolayer thickness, we show that the reaction rate depends on electron tunneling from the electrode to the nanoparticle. The tunneling decay constant is ∼4.3 nm^-1, which is small compared to those in literature for alkanethiols. We attribute it to a reduced tunneling barrier resulting from biasing the electrode potential negatively to the hydrogen reduction regime. In addition to allowing study of electron transfer of single nanoparticles, the work demonstrates an optical method to measure charge transport in molecules electrically wired to two electrodes.

Modulating and probing the dynamic intermolecular interactions in plasmonic molecule-pair junctions

The intermolecular interactions, including hydrogen bonds, are electromechanically modulated and probed in metal–molecule pair–metal junctions.